The cellular and functional changes which occur in the remaining kidney following contralateral nephrectomy have been the subject of much recent investigation. The basic mechanism responsible for these changes remains unclear, although evidence now points in the direction of a serum factor, which may be responsible for at least the early cellular and hemodynamic changes. This serum factor has been shown to be non-species specific and we have been able to detect its presence in human kidney donors and transplant recipients. The basic objective of the proposed research is to further define and characterize this serum factor in terms of its chemical structure. It will be necessary to develop an assay system which detects the presence of the serum factor, and which is more rapid and convenient than the one which is now available. It is proposed to use kidney cells in tissue culture for this purpose. The renal cell cultures will be incubated with multiple fractions of sera and analyzed for variance in mitotic activity. In this way it is planned to isolate the active fraction.